Traditional industrial processes for synthetic polymer microfiber spinning can be classified as either solvent-based or melt-based. As the name implies, solvent processing involves the spinning of a polymer solution with solidification of the fiber either through coagulation in a non-solvent (wet-spinning) or solvent evaporation (dry-spinning). In contrast, melt spinning produces fibers via the spinning of molten polymer that solidifies upon cooling; drawing usually accompanies this melt-based process to induce chain orientation and enhance mechanical properties. Typically, accessing nanoscale cross-sections is difficult, where fiber diameters of only 10-20 μm are achieved with applications in the textile industry. Pushing the limits of fiber production to the nanoscale has garnered recent attention in the processing arena.
Electrospinning is perhaps the most well-known, and one of the oldest techniques for generating sub-micron fibers in lab-scale from a polymer solution, or less commonly, a polymer melt via the application of a large electric field. This charged polymer jet is subjected to electrostatic forces, which act to elongate, thin, and solidify the polymer fiber in the characteristic “whipping instability” region. Although some success has been achieved with electrospun fibers in high-value added applications, such as air filtration, topical drug delivery, and tissue engineering scaffolds, significant disadvantages are low throughput and scalability. Additionally, electrospinning necessitates large volumes of toxic solvents that must be recovered by specialized equipment to make the process viable on a large scale.
Other approaches to nanofiber fabrication have emerged, including rotary-jet spinning, gas jet blowing, melt blowing, and bicomponent fiber spinning. Recent advances in rotary jet spinning have focused on a melt-based approach to nanofiber production with throughputs significantly higher than electrospinning, but improvements are ongoing to address processing complexity as it relates to broad applicability to a range of polymer systems. Melt blowing is a particularly commercially relevant and scalable technique for achieving fiber diameters on the order of tens of microns and higher; in this process, fibers are generated in-line by extrusion of a polymer through a die orifice, while a hot air jet blows down the extrudate. It is process compatible with a wide range of polymers, and is a solvent-less and environmentally-friendly manufacturing method. However, the pursuit of nanoscale fibers has been limited primarily to polypropylene for air filtration. Collectively, these limitations on nanofiber scalability increase manufacturing costs and lower productivity.
Polymeric materials have become ubiquitous in regenerative medicine as scaffolds for cell-seeding, where they have found application in the induction of cellular adhesion, proliferation, and differentiation. Nano-fibrous scaffolds are of particular use as they are porous, allowing transport of nutrients and waste products, have high surface area to volume ratios, and can provide directed cell growth based on fiber alignment. Synthetic fibers for regenerative medicine are usually comprised of polyesters, often poly(lactic acid) (PLA), poly(lactic-co-glycolic acid) (PLGA) or poly(caprolactone) (PCL), due to their degradability via hydrolytic pathways and resultant non-toxic byproducts. However, most polymeric scaffolds are unable to promote biological effects, as synthetic polymers do not possess the biochemical cues that are necessary to impact a cell's fate.
Modification of these polyester fibers typically relies on the degradation of the polymer chains, either through hydrolysis to expose carboxylic acids and alcohols, or through aminolysis to expose a second functional group off of the amine. Both of these routes degrade the polymer, potentially resulting in reduced mechanical properties and increased erosion of the fibers. Recent work has aimed to ameliorate this through the synthesis of telechelic polymers, which could then be processed into a scaffold.